This invention relates to cathode ray tube ("CRT" ) display of radiographic and other images of critical contrast detail by computer-controlled technique, and more particularly, to an improved system for measuring the absolute luminance of the CRT display and the actual effective luminance as would be measured from the viewer's perspective due to additional reflected ambient illuminance in order to enable closed loop adjustment of the display device and to thereby facilitate image perception and ascertain fidelity of image presentation conveniently over time. Such absolute measurements of device luminance performance and viewing environment can be reported to the presenting computer for optimization of task dependent image processing and quality control.
In the display of radiologic images by CRTs of a computer network as in a medical institution, whether by one display terminal or by any of multiple display terminals of a computer network, digital representations of images from various digital modalities are stored in computer memory and on computer media. The image representations are retrieved and displayed in any of various locations where there is such a display terminal for presentation, consultation, analysis, interpretation, and/or diagnosis. The digital representation of an image is converted to a matrix of quantified (pixel) values which ultimately determine the luminance of a uniquely associated area of the CRT face. The image is maintained for multiple seconds. The fidelity of image representation is inherently well-controlled by nature of its digital embodiment and conventional digital system practices. However, upon transformation to a luminous embodiment and presentation to the human visual system, the perceived image may differ from the "true" image as control for this final step has historically remained far less stringent. The ultimate consequences that result from suboptimal control range in significance from lowered productivity to missed pathology.
Two general display effects which perturb or delay the fidelity of transfer from pixel image to successful perception of subtle contrast detail are:
1) The CRT display device may be installed and certified to operate at an established luminance specification. Over time (which may vary from weeks to years depending on usage and design compromises) the effective luminance performance will degrade in response to factors such as "aging" of the cathode, phosphor, and glass (browning). It is desirable to be able to compensate for these inevitable CRT effects automatically.
2) The ambient light levels where the CRT display device is being used may be variable. It is not practical to always set room ambient lighting levels to accommodate the CRT display intensity for optimal image perception. The opposite may be desired: One wishes to establish display intensity, within the capability of the display device, as will maximize image fidelity by compensating for ambient lighting levels determined by a more critical task. As a simplified example, in a darkened room where ambient light level is low, minimum display luminance need be selected so that image areas of differing low brightness may be discerned without creating the perception of false contours while areas of higher brightness do not coalesce unnecessarily. In a more brightly illuminated surround, image areas of differing low luminance may fail to be discerned because of degraded contrast, even when image areas of higher brightness are entirely and accurately readable. It is desirable to present the image for maximum perceived contrast commensurate with the luminance specification of the CRT device and the level of ambient illuminance.
One scientifically based method for representing the absolute luminous performance of a CRT display device is the `characteristic curve` exemplified by the dashed line graph in FIG. 1. Here, the X axis represents the constant video drive level presented to the device over a display area and the Y axis represents the luminance value measured at the corresponding display area. Typically, the video drive level is a voltage between 0 volts (black) and 0.7 volts (white) or whatever video range is provided by the computer image source; here the video signal level is represented by an 8 bit digital value from 0 to 255. The absolute luminance is measured with a calibrated photometer (here in candelas per square meter) and plotted logarithmically to facilitate the comparison with the photopic brightness (perceptual) response of the human visual system to luminance (approximately logarithmic). Note that at drive levels where the graph takes on a more horizontal slope the luminance differences for equal changes in drive become correspondingly difficult to perceive. The first effect (display device does not meet luminance specification) can be readily determined from the endpoints of the graph, the maximum and minimum luminance of the display device. The solid line graph represents the characteristic curve of the same display device with ambient light included at the face of the CRT such that the resulting component which is reflected from the face (glass and phosphor) of the CRT over the area being measured by the photometer is not negligible. A measure of the second effect (luminance distortion from ambient light) is readily distinguished by comparing the graphs.
Accordingly, the goal is automatically to determine the luminous response of the monitor and then adjust CRT levels to ensure perception of all displayed pixel level differences (particularly in the "dark" areas) which might otherwise be obscured by ambient light to an accommodated viewer while maintaining an adequate overall contrast ratio of the display device. In the absence of such fidelity of image presentation, the medical professional looking at the image may be presented with a risk of missing critical details of the image. For example, the slight but clinically significant shading within a radiological image might not even be perceived.
Heretofore, users of CRTs who must routinely interpret images with critical contrast detail have resorted to six established methods to achieve luminance quality control, each of which addresses one of the two previously listed effects to thereby assure image fidelity. Each contributing method, the effect addressed, and its principal limitation(s) are outlined:
1) Manual calibration of the CRT, usually involving an external luminance photometer and readjustment of CRT gain and bias controls. This process can effectively control the first listed effect but requires maintenance personnel and external equipment to obtain a series of absolute measurements. Schedules for such maintenance on a preventive basis are not always considered or followed. Nonetheless, this is the "gold" standard for determining absolute luminance calibration of a CRT display device.
2) Manual adjustment of CRT brightness and contrast controls for "zeroing in" image adjustment with unconscious consideration for ambient lighting, as based upon the viewer's subjective determination of what appears to be needed. Such a manual method is often effective against the second listed effect but time-consuming; for any particular image, the viewer must perturb the controls to be sure that the settings are in fact optimal.
3) Measure ambient light with a photometric sensor mounted in the vicinity of the CRT and compensate the net brightness of the displayed image. This method attempts to compensate for the second listed effect automatically. In television display by CRT technique, it has been known for many years to use photocell devices for directly sensing room ambient light and for automatically correcting CRT brightness or intensity drive levels in response to sensed changes in ambient light levels. Such television-related technology is evidenced by U.S. Pat. Nos. 2,264,172; 3,153,172; 3,165,582; 3,214,517; 3,471,740; 3,649,755; 4,589,022; 4,769,708; and 4,799,107. Technology for the demanding ambient of a cockpit is addressed with multiple sensors in U.S. Pat. Nos. 5,057,744; and 5,270,818. These prior ambient light-sensing arrangements are not believed to be fully suitable for CRT display of radiologic and other static images displayed by computer-controlled technique for critical interpretation because a direct measurement of ambient light level, as by using a photocell exposed through an aperture to the room ambient, as heretofore known, may not adequately sense the true illumination impinging over the CRT face. Additionally, the sensor's independent collector is subject to accumulation of dust which will effect its response and, without appropriate filtering to match the spectral response of the human eye, the system may over-respond to infrared or other invisible radiation as is prominent in the spectrum of some artificial lighting.
4) Maintaining a fixed ratio of the brightness of a displayed image to the ambient brightness to effect a fixed image contrast ratio by closed loop feedback to the video amplifier gain. This technique is intended to simultaneously compensate for variations in both the ambient illumination and display system luminance performance. Such a constant contrast system is evidenced by Newman in U.S. Pat. No. 3,649,755. Such a system must position the logarithmic sensor to receive the identical proportion of image brightness to ambient brightness as would be seen by the viewer who would observe the contrast ratio as invariant. Such a position is only certain to be at the same nominal place as the observer's eye and thus is obtrusive. Such a system depends on an approximated definition of contrast C=(Bc+Ba)/Ba which will be problematic as the ambient brightness (Ba) goes to 0. The absence of ambient brightness is not an uncommon situation for critical contrast viewing. Gain adjustment of such a system is not only a function of the luminance performance of the device "hardware" and the ambient illuminance but also depends on the image "software" being displayed (a uniformly light gray image will be presented indistinguishably from a uniformly dark gray image). Such an "editorializing" system may not be compatible for the presentation of processed digital images which have already undergone optimization. Additionally in both methods 3 and 4, with the absence of any absolute measure, the viewer is left to visually determine that the combination of ambient compensation and CRT performance have exceeded a threshold such that the true image may be compromised or unattainable.
5) Analysis of the pixel data to calculate a value which predicts the average image luminance and is used to adjust the overall luminance relative to the ambient. Such method is evidenced in U.S. Pat. No. 4,952,917 and addresses effect 2. This method requires calculating means for predicting image luminance in advance of display. Once again, any continuous modification of device performance based on image content at the display device may preclude benefit from previously computed image enhancement.
6) Use of CRTs with very high brightness capability can minimize the deleterious interaction from ambient light. A fixed contrast ratio can be maintained over a wider range of ambient illuminance than with a CRT display device of lower available brightness. Such method can successfully overwhelm the second listed effect for an extended range of illuminance. The method can be expensive to implement. Viewer performance may not be enhanced if luminance is increased without maintaining contrast. Since brightness is roughly a logarithmic function of luminance, substantial CRT luminance increases must be achieved to effect modest contrast improvement.
The existing methods are not adequate to the goals of wide scale deployment (low cost), increased productivity (automatic operation) and absolute measure (quality control) for high fidelity presentation to readers of images with critical contrast detail.